[PATCH 1/2] utils: raspberrypi: ctt: Fix NaNs in lens shading tables

[Naushir Patuck]

Hi David,

On Mon, 28 Apr 2025 at 11:36, David Plowman
<david.plowman at raspberrypi.com> wrote:
>
> The problem occurs when the calculation could lead to a final row (or
> column) of grid squares with no pixels in them (and hence, NaNs).
>
> One specific case is a Pi 5 with an image width (or height) of 1364,
> so that's 682 Bayer quads. To give 32 grid squares it was calculating
> 22 quads per cell. However, 31 * 22 = 682 leaving nothing in the final
> column.
>
> The fix is to do a rounding-down division by the number of cells minus
> one, rather than a rounding-up division by the number of cells. This
> turns the corner case from one where the final row/column has no
> pixels to one where we don't quite cover the full image, which is how
> we have to handle these cases.
>
> Signed-off-by: David Plowman <david.plowman at raspberrypi.com>

Reviewed-by: Naushir Patuck <naush at raspberrypi.com>

> ---
>  utils/raspberrypi/ctt/ctt_alsc.py | 7 ++++---
>  1 file changed, 4 insertions(+), 3 deletions(-)
>
> diff --git a/utils/raspberrypi/ctt/ctt_alsc.py b/utils/raspberrypi/ctt/ctt_alsc.py
> index 1d94dfa5..f4fd09e3 100644
> --- a/utils/raspberrypi/ctt/ctt_alsc.py
> +++ b/utils/raspberrypi/ctt/ctt_alsc.py
> @@ -127,11 +127,12 @@ def alsc(Cam, Img, do_alsc_colour, plot=False, grid_size=(16, 12), max_gain=8.0)
>      channels = [Img.channels[i] for i in Img.order]
>      """
>      calculate size of single rectangle.
> -    -(-(w-1)//32) is a ceiling division. w-1 is to deal robustly with the case
> -    where w is a multiple of 32.
> +    The divisions here must ensure the final row/column of cells has a non-zero number of
> +    pixels.
>      """
>      w, h = Img.w/2, Img.h/2
> -    dx, dy = int(-(-(w-1)//grid_w)), int(-(-(h-1)//grid_h))
> +    dx, dy = (w - 1) // (grid_w - 1), (h - 1) // (grid_h - 1)
> +
>      """
>      average the green channels into one
>      """
> --
> 2.34.1
>